Electro-optical circuits are widely used in communication and sensing devices. In particular, optical filters that rely upon layers of alternating materials having different refractive indices providing constructive and destructive optical interference are widely used. Such filters can transmit or reflect particular frequency ranges of electromagnetic radiation.
Proximity sensors are sensors that detect nearby objects and are commonly used in smartphones and for industrial applications to measure the distance between machine components. In one implementation, proximity sensors include a light emitter and a light sensor that senses the light emitted and measures the time between light emission and sensing light reflected from an object to determine a distance to the object that depends on the speed of light. The light emitted is typically infrared radiation invisible to human beings, such as light from an infrared light-emitting diode (LED). However, light sensors, such as silicon photodiodes or silicon phototransistors are generally responsive to a wide range of light frequencies. In order to improve the signal-to-noise ratio of the sensed signal, an optical filter is provided that substantially transmits only the specific frequencies of the emitted light so that the light sensed by the light sensor is limited to the emitted light.
Sensors with optical interference filters are conventionally constructed by providing a semiconductor substrate, such as silicon, and photolithographically forming a light sensor in the semiconductor substrate. Optical filter layers are blanket deposited and then patterned over the light sensor using resist lift-off methods. Alternatively, a shadow mask is provided with an opening positioned over the light sensor. Multiple layers of different materials are deposited over the shadow mask and the opening over the light sensor, for example by evaporation, to form the optical filter on the light sensor. The shadow mask is then removed. Other patterning methods are also possible, such as etching. For example, U.S. Pat. No. 8,803,068, U.S. Pat. No. 8,624,341, and U.S. Pat. No. 8,598,672 describe patterned interference filters deposited on a silicon substrate using sputtering, vapor deposition, electrochemical deposition, molecular beam epitaxy, and atomic layer deposition, all of which provide optical layers that are essentially welded or fused to or form co-valent bonds with the underlying substrate. Other structures using multiple substrates with molded or cast interference filters are known, for example as taught in U.S. Pat. No. 8,143,608.
However, this process is very wasteful, since only the portion of the optical filter layer materials that remains after lift-off or that pass through the shadow mask is used. Furthermore, the process is sequential, as the light sensor must be made first before the optical filter is applied over the light sensor, increasing the elapsed manufacturing time. Thus, if multiple optical filters are formed on a common substrate, the deposition process must be repeatedly and sequentially performed, leading to further waste in materials and time.
There is a need, therefore, for improved integration of optical filters in electro-optical applications and devices.